What it does
Food tests are the simplest way to find out which biomolecules a sample contains. Each test uses a reagent that changes colour with one kind of molecule:
- Iodine test for starch: orange-brown turns blue-black.
- Benedict’s test for reducing sugars such as glucose: blue turns green, yellow, orange or brick red when heated, depending on how much sugar there is.
- Biuret test for proteins: blue turns violet.
- Sudan III test for fats and oils: a red-stained oily layer floats on top.
This page is a virtual bench. Choose a sample (water, glucose, sucrose, starch, egg white, oil, milk, potato or groundnut), choose a test, add the reagent, heat it if it needs heating, and watch the tube. Each finished test goes into a results table. Then try the mystery sample: test it four ways and tick what it contains.
It fits Class 8 and Class 9 Science (food, nutrients and their tests), Class 10 (life processes: nutrition) and Class 11 Biology (biomolecules: carbohydrates, proteins and lipids, and why sucrose is not a reducing sugar). After this, see how proteins fold in protein structure levels and how enzymes break food down in enzyme activity lab.
How to use it
- Pick a Sample from the list. For glucose you can set its strength from 0 to 2.5 %.
- Pick a test: Iodine, Benedict’s, Biuret or Sudan III. The line underneath says what it finds and what to add.
- Press Add reagent. For Benedict’s, then press Heat in water bath.
- Read the result under the tube. It is added to the Results table.
- Press Fresh tube to start again with the same sample and test.
- For the Mystery sample, press the button, run all four tests, tick what you think it contains, and press Check my answer.
Worked example
Choose Glucose solution at 1.0 % and Benedict’s. Add the reagent: the tube is blue and nothing happens, because Benedict’s needs heat. Heat it: the colour moves through green to orange, a fair amount of reducing sugar. At 0.3 % it stops at green (a trace), and at 2.0 % it goes all the way to brick red.
Now choose Sucrose. Heated with Benedict’s it stays blue, because sucrose is not a reducing sugar. Tick First boil with dilute acid, then neutralise and test again: the acid splits sucrose into glucose and fructose, and the result turns brick red.
Test Milk with all four: iodine stays orange-brown (no starch), Benedict’s goes orange (lactose is a reducing sugar), biuret goes violet (protein) and Sudan III shows a red layer (fat).
Tips
- Always run the water control with each test. It shows the reagent’s own colour, so you can tell a real change.
- Benedict’s is semi-quantitative: the colour gives a rough amount, not an exact one.
- Use the same volume of sample each time, so results can be compared fairly.
FAQ
Why is it called a “reducing” sugar? Glucose and similar sugars reduce the blue copper(II) ions in Benedict’s solution to red copper(I) oxide, which makes the colour change.
Is the emulsion test for fat the same as Sudan III? No, it is another fat test: shake the sample with ethanol, then add water; a milky white emulsion means fat. Both are used in schools.
Are these colours exact? No. They are drawn to look like typical results; real ones depend on the reagent, the amounts and the heating time.
Where the numbers come from
- The iodine, Benedict's, biuret and Sudan III tests, their colours, and which common foods contain starch, reducing sugar, protein or fat: general school biology. Colours are drawn approximately.
- Benedict's colour scale (green about 0.1 to 0.5 %, yellow 0.5 to 1 %, orange 1 to 1.5 %, brick red above about 1.5 % reducing sugar) is the usual rough guide, rounded; real colours depend on the reagent, time and heating.
- Checked in the tests: every sample and test gives the expected positive or negative result, the Benedict's scale changes at 0.5, 1 and 1.5 %, and 2 % of 50 mL is 1 g. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.